The Analytical Approach Edited by Claude A. Lucchesi
Characterization of Corrosion Scale from Nuclear Reactors: Approaching the Whole Problem L. D. Hulett, John M. Dale, H. W. Dunn, and P. S. Murty Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tenn. 37830
Corrosion is a serious problem t h a t affects the cost, safety, lifetime, and overall practicality of nuclear reactors. Analytical chemists associated with nuclear energy research are heavily in volved in the solution of corrosion problems. To properly do their jobs, they m u s t assume more responsibility than just t h a t of analysts. Since engi neers or other scientists in charge of the corrosion studies often do not know how or when to apply the neces sary analytical techniques, it is the re sponsibility of analytical chemists to help coordinate t h a t part of the study. By doing this, they generate an extra dimension to their work by becoming active, decision-making parts of the overall research efforts. T h e y become even more a part of the action if they interpret results and a t t e m p t to relate t h e m to the cause of the problem. T h i s is what we mean by the "whole prob lem approach". For the case history which follows, the whole problem ap proach assisted in determining the cause of scale t h a t formed on the in side of a steam generator being tested for use with a nuclear reactor. Corrosion scales must be character ized to determine their causes and possible prevention. This requires many different types of analytical techniques, and the information from each examination must be assimilated with t h a t from the others so t h a t a
general description of the corrosion can be constructed. T h e characterization questions posed are as follows: (1) W h a t elements are present? (2) W h a t are the chemical states (compound or ionic forms) of the ele ments? (3) How are the elements and com pounds distributed? Is the specimen homogeneous or heterogeneous? (4) W h a t are the sizes of the parti cles t h a t make up the specimen? (5) How do the particle surfaces differ from their interiors? (6) W h a t is the origin of the speci men (precipitation, corrosion,. . .)? To collect this much information with the same solid sample and to pre serve the integrity of the sample dur ing analysis, nondestructive tech niques m u s t be used. T h e x-ray and electron physics methods fit these re quirements quite well and were ap plied t o this study of scale on an inconel (alloy 600) corrosion specimen from the inside of a steam generator. T h e methods used and their sequence of application are shown in Figure 1. T h e first step in the examination (Figure 1) was to chip a small piece of the scale from the inconel substrate and analyze it by x-ray induced x-ray fluorescence ( X R - X R F ) to answer characterization question 1. This piece contained the elements of the sub
1160 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
strate Ni, Cr, and Fe, b u t the propor tion of iron was much higher. Also ob served was bromine in a rather low concentration of no more t h a n 0.1%. T h e next step was to m o u n t a portion of the specimen in epoxy and to cut and polish it so t h a t the scale and in conel substrate could be viewed in cross section by scanning electron mi croscopy to answer questions 3 and 4. Figure 2 shows a low-magnification scanning electron micrograph of the scale and substrate. Note t h a t the outer portion of the scale is of consid erably different texture from the inner regions. T h e overall thickness of the scale is 150 μτη. T h e outer region of the scale, of different texture, is about 15 μπι in thickness. X-ray fluores cence, induced by the beam of the mi croscope, was used to analyze various parts of the scale shown in Figure 2 and to answer characterization ques tion 3. Phosphorus, silicon, and small amounts of calcium were found in ad dition to iron, chromium, and nickel. Isolated inclusions with high concen trations of manganese were also found in the scale. Another very significant observation was t h a t the outer 15 μτη of the scale, which had a different texture, contained predominantly Ni and Fe. Cr and Ρ were the only other elements present in the scale, and their concentrations were very low. T h e summary of conclusions after
Figure 1. General approach to analysis of complicated mixtures of solids X R - X R F and S E M - X R F examination is shown in Table I. ESCA was then used for examining the scale. After the outer surface of the scale was analyzed, it was scraped away with a gold knife edge so t h a t a thin layer of scale remained at the alloy scale interface. T h e ESCA spectrum of the scale a t the alloy interface (Figure 3) was essentially the same as t h a t on the outer surface; only one of the three transition metals, nickel, was detected. Iron and chromium, known from X R F data to be present in the scale, are not revealed in the photoelectron (ESCA) spectrum. T h e explanation for this anomaly is t h a t the surfaces of the oxide crystallites t h a t make up the scale are very different from their interiors. In addition to nickel, Figure 3 shows t h a t sodium, phosphates, bromates, and silicates are present on the surfaces of the scale particles. T h e oxidation states of the phosphorus, bromine, and silicon were deduced from their chemical shifts. T h e ESCA study combined with X R - X R F applies to characterization question 5.
Another interesting observation from the ESCA studies was t h a t carbon was present in two different oxidation states (Figure 4). T h e carbon spectrum of the scale is compared with t h a t of N1CO3. In both spectra of Figure 4 the peak having the higher kinetic energy is due to hydrocarbon vapors condensed from the air and the atmosphere of t h e spectrometer. T h e peak having lower kinetic energy is due to carbon in the form of carbonate. T h e carbonate form has slightly • higher binding energy than t h a t of hydrocarbon and releases its photoelectrons with lower kinetic energy. Some of the ESCA spectra of carbon on the scale showed a third carbon peak having even lower kinetic energy than the carbon of nickel carbonate. T h e chemical shift of this peak matched t h a t of Na2CO : j. Since sodium was found on the scale surface, we assumed t h a t the third carbon was from Na2COs. T h e conclusions from the ESCA study of the scale are indicated in the second entry of Table I.
was applied to answer characterization question 2. T h e scale was scraped from the inconel substrate for these studies. T h e only pattern t h a t could be interpreted was t h a t of a spinel. Spinels involving Ni, Fe, and Cr are well known. However, there are three different combinations of these ele-
After the ESCA examination of the scale, the x-ray diffraction technique
Figure 2. Scanning electron photomicrograph of scale
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Table I. Summary of Conclusions XR-XRF, SEM-XRF Examination Fe, Cr, Ni, Ρ, Si, Ca, and Br are present in scale. Ni and Fe predominate in outer 15 (urn of scale. Mn inclusions in scale ESCA Examination of Scale There is a coating on particles that make up scale. Composition of coating is greatly different from that of scale particles Main components of particle coating are phosphates, bromates, silicates, nickel carbonate, and sodium carbonate X-ray Diffraction Examination of Scale N i x F e 3 - x 0 4 is main component of particles that make up outer 15 Mm of scale Ni x Cr 3 _ x 0 4 and Fe x Cr 3 _ x 0 4 are main components that make up particles in interiors of scale Ion Etch-ESCA Examination of Scale Alloy Interface Corrosion process causes enrichment of Cr in scale Origin of Scale Whatever corrosion process that is taking place is causing a Cr enrichment at alloy-scale interface Most of scale is due to precipitation rather than corrosion Solid-state transformation is taking place at outer surface of scale, causing formation of Ni x Fe 3 _ x 0 4
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Figure 3. Photoelectron (ESCA) spectrum of surface of scale crystallites ments t h a t form spinels with essen tially the same diffraction pattern: N i x F e 3 _ x 0 4 , N i x C r 3 - x 0 4 , and Fe x C r 3 _ x 0 4 . T h e N i - F e spinel is magnet ic. When we examined the scale scrap ings, we found a magnetic component which was easily separated from the rest of the scale. When an x-ray dif fraction pattern was obtained for the magnetic and nonmagnetic compo nents, the spinel structure was found
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for both. At this point, it was recalled from t h e S E M - X R F d a t a t h a t Ni and Fe were the predominant elements found in the outer 15 μιη of the scale, and it was concluded t h a t the outer 15 Mm of the scale was the N i x F e 3 _ x 0 4 spinel, while N i x C r 3 _ x 0 4 and Fe x C r 3 _ x 0 4 compounds were present in the other parts of the scale. These conclusions are listed in the third entry of Table I.
Figure 4. Comparison of carbon photoelectron peaks of corrosion scale with those of N1CO3
Figure 5. Concentration profiles of Ni, Cr, and Fe at scale-inconel interface
T h e scraping of the scale with a gold knife edge left a thin film of scale next to the inconel substrate. It was felt t h a t an examination of this layer might yield information about the corrosion process t h a t occurred at the alloy interface, and the ESCA-ion etch technique was applied at this point. This involved the bombardm e n t ("sputtering") of the surface with 1000-eV argon ions followed by photoelectron spectra measurement. T h e ion etch t r e a t m e n t removed material in a controlled fashion so t h a t the composition of the scale as a function of depth could be measured. T h e results of this study are plotted in Figure 5. Note t h a t the composition of the scale tends toward about 7% Ni, 5% Cr, and 1% Fe. This composition indicates t h a t the scale at the interface is enriched in Cr due to the corrosion process. If the three metals had been in the scale in the same proportions as for the substrate, their percentages would have been something like 7.8% Ni, 1.4% Cr, and 0.8% Fe. This conclusion is also summarized in Table I. T h e quantitative interpretation of the ESCA data used for Figure 5 was done according to the scheme suggested by Carter et al. (2). T h e first four conclusions of Table I regarding the character of the scale are summarized schematically in Figure 6. We shall now address characterization question 6, often the most important of all from the customer's point of view: What is the origin of the scale? A comparison of the thickness of the scale with the mass loss (decrease in thickness) of the substrate indicates t h a t the mass of the scale is too great to be accounted for by corrosion. A large portion of the scale must therefore be due, not to corrosion, but to precipitation from the boiler environment. Our characterization studies support this conclusion. T h e manganese inclusions must necessarily have come from precipitation since manganese is not a component of inconel. T h e phosphate was probably from corrosion inhibitors p u t into the boiler. Calcium, sodium, silicate, and bromate must have come from precipitation also. T h e Ni x Fe3-*04 forms a clearly distinct phase from the rest of the scale. This suggests t h a t a solid-state transformation is taking place at the outer layers of the scale.
References (1) W. J. Carter, G. K. Schweitzer, and T. A. Carlson, J. Electron Spectrosc. Relat. Phenom., 5,827(1974).
Figure 6. Overall character of scale 1164 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Oak Ridge National Laboratory is operated by the Union Carbide Corp. for the U.S. Energy Research and Development Administration.
